As the complexity of semiconductor integrated circuits grows and as the circuit densities defined on semiconductor integrated circuits increase, it becomes increasingly important to characterize accurately the precise nature of the semiconductor wafer from which integrated circuit chips are made. For example, silicon wafers made by the liquid encapsulated Czochralski process typically contain trace interstitial oxygen of a concentration of typically 1.times.10.sup.18 atoms per cubic centimeter. These trace impurities are not necessarily harmful and, in fact, are sometimes used by process engineers as "gettering" sites, that is, sites for neutralizing other unwanted impurities. However, since too great a concentration of interstitial oxygen would harm device performance, it is common to specify the range of concentration or density of interstitial oxygen required in the wafers to be used.
One way of measuring interstitial oxygen in a silicon wafer is to direct light of an appropriate wavelength through the wafer. Since interstitial oxygen absorbs light of a characteristic wavelength, its concentration can be measured by measuring the proportion of light at that characteristic wavelength transmitted through the wafer. A drawback of this method is that it cannot be used for measuring interstitial oxygen in highly doped silicon wafer because such doping tends to make the wafer opaque. "Doping" refers to the impregnation of the silicon crystal with impurities such as phosphorous or boron which determine its current-carrying capabilities. Vendors of silicon wafers often measure the interstitial oxygen in an undoped wafer and then assume that the interstitial oxygen concentration in a highly doped wafer made by the same crystal growth process will be the same. Unfortunately, this assumption is often not correct. In fact, papers in the literature indicate that interstitial oxygen concentration is often changed by the type and concentration of conductivity-determining impurities used in doping the wafer.
Highly doped silicon wafers can be tested for interstitial oxygen by various destructive techniques known in the art, such as secondary ion mass spectroscopy (SIMS), charged particle activation analysis (CPAA), or gamma ray activation analysis. For the sake of brevity, these techniques will not be reviewed, but they are all generally unsatisfactory in that they require destruction or some sort of structural damage to at least part of the wafer in order to determine interstitial oxygen concentration. There has, therefore, been a recognized need in the art for a method to detect, conveniently, accurately and non-destructively, trace interstitial oxygen in silicon wafers, especially highly doped silicon wafers, i.e., wafers having a doping concentration in excess of about 1.0.times.10.sup.18 conductivity-determining atoms per cubic centimeter.